In recent years, with rapid proliferation of network services typified by the Internet and mobile phones, realization of more advanced communication services is expected. A so-called handover function and a mobility management function that allows continuity of communication when a user moves while carrying a terminal have been provided for the mobile phones, and by further developing and enhancing such functions, realization of, for example, a function is demanded that can allow continuity of communication even if the terminal is switched during communication.
As a communication network expands and develops to enable connection of various terminals to the network, a user will tend to have various kinds of communication terminals that can connect to the communication network. It is expected that these communication terminals include not only portable terminals such as mobile phones, but also stationary apparatuses installed in homes such as television sets, which are expected to work also as communication terminals.
On a communication network side, on the other hand, by providing a common IP backbone making use of an IP (Internet Protocol) communication technology constructed with the development of the Internet, it becomes easier to enjoy services common to various kinds of communication terminals without being aware of differences in access means such as wireless and cables, network operators, and service providers.
Under such circumstances, a user freely uses different communication terminals owned by the user such as a mobile communication terminal, a PDA, and a personal computer for different purposes depending on user's situation and the like and even if the same service is used, a desire naturally arises to use the service using a terminal carried or selected by the user at that point. Moreover, there is assumed a situation where a terminal be preferably switched to another terminal during communication. In such a situation, a user's desire to seamlessly switch to another terminal without interruption while maintaining a communication state, instead of once disconnecting a communication and establishing another connection from another terminal, needs to be realized. More specifically, there is the case in which image content by streaming delivery has been viewed on a personal computer at home and when going out, a receiving terminal is switched to a mobile communication terminal or PDA to continue to view the same content on the terminal to be carried outside. Also the case can be considered in which, since a user gets home while communicating with an opposite party with a TV phone, the communication with the opposite party with the TV phone is continued by switching to the TV set at home.
To realize a seamless terminal switching as described above, when receiving multicast delivery, a terminal switching can be realized relatively easily because data of the same content is simultaneously delivered to a plurality of (unspecified number of) communication terminals from a terminal on a transmitting side and a terminal of switch destination needs only to connect to the terminal on the transmitting side so that the data is also received by the relevant receiving terminal.
However, in the case of unicast delivery in which content is delivered to specific terminals or one-to-one communication, a procedure is needed which instructs a terminal on the transmitting side to once suspend a communication and, connects a terminal of switch destination to the terminal on the transmitting side, and then resumes the communication after notifying that the terminal on a receiving side has been switched.
FIG. 6 shows an example of terminal switching in a conventional control system of a communication network. The example in FIG. 6 shows the case in which a user switches a communication terminal #1 currently in use for communication to a communication terminal #2. First, the user sends control information denoted by an arrow 101 from the communication terminal #1 currently receiving data to a server apparatus X on the delivery side to instruct to suspend the delivery. Next, control information denoted by an arrow 102 that requests to disconnect communication connection to the server apparatus X is sent to a control apparatus Y in the communication network to disconnect the communication. Then, when the user sends out a connection request to the same server apparatus X as described above denoted by an arrow 103 from the communication terminal #2, which is a terminal of switch destination, to the control apparatus Y in the communication network, the control apparatus Y sets up a communication path between the communication terminal #2 and the server apparatus X in the communication network to set a connection between apparatuses concerned. Subsequently, when control information to instruct resumption of delivery denoted by an arrow 104 is sent out from the communication terminal #2 to the server apparatus X, the server apparatus X sends out data from a point where the delivery was suspended and the user can receive data to be continued using the communication terminal #2, which puts terminal switching in practice.
Here, address information of the server apparatus X needed for the communication terminal #2, which is the terminal of switch destination, to connect to the server apparatus X is specified in a format of a telephone number or URL by means of input through the user's operation of the communication terminal #2 or the like. In this example, the control apparatus Y in the communication network is in charge of only connection and disconnection between the communication terminal #1 or #2 and the server apparatus X and has no control function to switch terminals, and thus the terminal switching is realized by functions of the communication terminals and server apparatus to interrupt and resume a communication.
FIG. 7 shows an example of another conventional method of terminal switching. To perform terminal switching, the communication terminal #1 on the receiving side sends out, as denoted by an arrow 201, control information to the server apparatus X on a delivery side to instruct to suspend delivery. If, after holding a communication with the server apparatus X, a user performs a transfer operation, the communication terminal #1 sends out, as denoted by an arrow 202, control information to the control apparatus Y in the communication network to request to transfer a connection with the server apparatus X to the communication terminal #2. The control apparatus Y, which received the control information, originates a call to the communication terminal #2, as denoted by an arrow 203, by sending out control information to instruct to establish a connection to the server apparatus X to the communication terminal #2. When the communication terminal #2 answer the call, as denoted by an arrow 204, a connection is established by setting a communication path between the communication terminal #2 and the server apparatus X.
Subsequently, by transmission and reception of control information between the communication terminal #1, which is a transfer source, and the control apparatus Y, as denoted by an arrow 205, the connection between the communication terminal #1 and the server apparatus X is disconnected. At the same time, the communication terminal #2 sends out control information, denoted by an arrow 206, to request to resume the communication that has been put on hold to the server apparatus X to receive data to be continued. In this example, though the control apparatus Y in the communication network has a control function to transfer a call to another terminal, the control apparatus Y does not have functions to interrupt and resume data transmission/reception between the communication terminal and server apparatus so that continuity of communication, that is, a seamless switching is not guaranteed.    Non-Patent Document 1: “Special Feature: All-IP Mobile Network” by Yabusaki et al., NTT DoCoMo Technical Journal Vol. 10 No. 4 Jan. 2003, P. 6 to 34    Non-Patent Document 2: “Service Handoff Mechanism toward Realization of Ubiquitous Applications” by Imai et al., Collection of Drafts for the Fourth Symposium on YRP Mobile Communications for Exchange of Industry, Academia and Government, 2002, P. 112 to 113    Non-Patent Document 3: “Session Layer Mobility Support for Dynamic Network Services” by Kaneko et al., Collection of Drafts for the Fourth Symposium on YRP Mobile Communications for Exchange of Industry, Academia and Government, 2002, P. 114 to 115